In particular, the invention concerns an overspeed safety device applied on a pneumatic rotation tool which is equipped with a speed limiting means and which carries on its housing a maximum speed mark.
The present invention is very suitable, however not exclusively intended for portable grinding machine applications where too a high motor speed is a potential danger to the operator and others at the working site. On such machines high safety demands have been put up to minimize the risk of grinding tool bursting due to overspeed.
These safety demands are well founded, because when a grinding wheel is rotated at too a high speed the inertia forces will exceed the strength of the grinding wheel material and the grinding wheel will suddenly become disintegrated into a number of fast travelling pieces. Each of those pieces posseses a high kinetic energy and is extremely dangerous to people in the vicinity of the tool, especially to the tool operator.
Usually, grinding machines are provided with speed governors for adapting the power supply to the motor in response to the load applied on the machine. The main purpose of a speed governor is to limit the speed at no-load conditions.
One way to increase safety at portable grinding machines is to employ an overspeed release mechanism for shutting off the motor in case the speed governor should become ineffective in keeping down the speed.
This is a way to prevent a malfunctioning speed governor from allowing the motor speed to reach dangerous magnitudes. As long as the speed governor operates correctly the overspeed release mechanism remains inactive, but the moment the speed governor due to hard wear, seizure or both becomes incapable of keeping down the speed the release mechanism will come into action and stop the motor.
A criterium for obtaining disintegration of a grinding wheel is the peripheral velocity of the wheel, which is dependant on the rotation speed as well as the diameter of the wheel. This means that a grinding wheel of a certain diameter is allowed to be rotated at a greater rotational speed than a grinding wheel of a bigger diameter provided the material strength is the same in the two wheels. This also means that the speed governor and the overspeed release mechanism, if fitted, have to be carefully adapted to the size of the grinding tool, or oppositely a grinding machine provided with a speed limiting means intended for a certain speed level may not be fitted with a grinding tool having a diameter exceeding a certain critical diameter. This critical diameter is determined by the maximum speed of the machine and the material strength of the tool. Accordingly, it is very important from the safety aspect that the grinding wheel and the speed limiting means of the machine are carefully adapted to each other.
Today, grinding machines of the above described type are provided with a mark plate telling about the maximum speed of the machine, and, thereby, giving indirect information of the maximum size of the tool to be safely used with the machine.
A serious problem resides in the fact that the mark plate attached to the outside surface of the machine housing or data engraved in the housing may give false information about the maximum speed of the motor.
The problem is that to many grinding machines there are available speed governors and overspeed release mechanisms covering a wide range of speed levels, These speed limiting means are intended to be fitted into machine housings of identical design but carrying different maximum speed marks. There is an obvious risk, and it has happened, that a speed control unit designed for a certain maximum speed level has been inserted in a machine housing carrying a mark on which a lower speed is readable. To such an erroneously assembled machine there may be attached an oversized grinding wheel, and there is a great risk this wheel will explode.
This means that although the machine is equipped with a perfectly operating speed governor as well as an overspeed release mechanism as an extra safety means, there might still be a risk for a tool disintegration.